Functional electrical stimulation (FES) is a procedure whereby electricity is used to restore motor function in paralyzed or neurologically impaired patients. FES treatment of debilitated musculature in persons so afflicted has been found to improve the physical capacities of the person by at least partially restoring the function of the damaged muscles. By functional electrical stimulation, electrodes are used to excite musculature by either directly stimulating the muscle fiber or indirectly stimulating nerves which are physiologically connected to muscle fiber. The excited nerves serially discharge neurons which innervate groups of muscle fibers which contract in response thereto. In this manner, gross muscular tension may be caused by a series of excitation signals applied to numerous muscle fibers. The amount of muscle fiber which is stimulated by an FES system is referred to as the level of recruitment.
The manner in which electrodes in functional electrical stimulation systems transmit electrical pulses to the nerves depends upon the physical relationship of the electrodes and the nerve endings. The method of electrode attachment in turn depends on the type of electrode being used in the system. Several types of FES electrodes are known in the art, including (i) intraneural electrodes, (ii) cuff electrodes, (iii) intramuscular electrodes, (iv) surface electrodes, and (v) epimysial electrodes. Each of these types of electrodes is constructed and operates in a particular manner.
Intraneural electrodes are placed within the nerve a manner which actually penetrates the nerve to be stimulated, piercing the nerve in at least one point. Epineural electrodes are also employed to stimulate muscle controlling nerves but are attached directly to the outside of the nerve. Typically an uninsulated conductive portion of an intraneural electrode is placed within the nerve and transmits electrical signals directly to the nerve. Because intraneural electrodes are implanted directly within individual nerves, they are useful in controlling specific muscle fibers. Intraneural electrodes are implanted with precision to insure that the electrode pierces but does not damage the nerve in which it is implanted.
Cuff electrodes, also known as nerve cuffs, physically surround the nerves to which the electrical signals are to be sent. Typically, a cuff electrode has at least one section of conductive material which contacts an outer surface of the nerve. Electrical signals propagate from this conductive material to the nerve to effect contraction of the muscle to be stimulated. Nerve cuff electrodes have a variety of applications in that they may be attached to either nerves that stimulate single muscles or to nerve trunks with branches that are connected to several muscles.
Intramuscular electrodes are implanted within muscle fiber and typically consist of a coiled wire with a non-insulated tip which is located immediately adjacent to or near the nerves of the muscle to be stimulated. Because the electrodes are implanted within the muscle, the conductive areas of the electrodes are usually very small, often consisting of a wire or group of wires having bare ends. Although the wires are useful in controlling individual muscles because the electrical charge can be delivered to a specific areas, effective recruitment of groups of muscles over a larger area of implantation is often difficult to obtain.
Surface electrodes are electrodes which are typically attached to the surface of the skin near the muscle to be stimulated. To ensure consistency in operation, the location at which the electrodes are attached must be rather precise. Also, the surface electrodes must be held in place on the skin by adhesive which must be cleaned off thoroughly when the electrodes are removed from the skin. Surface electrodes require relatively high current as compared to implanted electrodes to successfully activate the nerve fibers that lead to muscle contraction. Moreover, the process of attaching and removing the electrodes is time consuming, messy, and difficult for many patients to perform themselves, requiring the assistance of another person, usually a trained clinician.
Epimysial electrodes are implantable electrodes which are mounted to the surface of the epimysium, which is a fibrous sheath which surrounds skeletal muscle. The epimysial electrodes are surgically implanted and typically sutured to the epimysium of the targeted muscle. The electrode must be located as close as possible to the motor point, which is the optimum point for stimulation, of the muscle to maximize the muscular response. In the case of large muscles in which the motor point may be located far from the surface of the muscle, stimulation of the motor point is difficult using epimysial electrodes. Failure to stimulate the motor point may only cause contraction of the fibers in the close vicinity of the electrode, resulting in insufficient contractions to restore functionality to the limb. Further increases in operational current to obtain greater recruitment may cause contraction of nearby non-targeted muscle fiber.
Each of the electrodes described above is directly or indirectly interfaced to either muscle fiber or muscular nerve endings, and each functions in a characteristic manner. None of the above described electrodes, however, provide an implantable electrode which does not directly penetrate or attach to the surface of muscle or muscle controlling nerves, need not be sutured into position, and which provides a conductive surface area large enough to stimulate the motor point of larger muscles within the body. The electrode of the present invention provides these features in a device which can be implanted in a safe, easy and reversible manner, while at the same time providing a long lifetime of highly consistent and predictable performance.